Nest Thermostat Error E35 — What It Means & How to Fix It

What Does This Error Mean?

Nest error E35 means your heating system is running but the indoor temperature is not rising as expected. The Nest detected a 'heating delta' problem — your furnace or heat pump is on, but the house is not warming up at a normal rate. This can be caused by a clogged filter, a failing heat exchanger, a cracked duct, or a heat pump that is too small for the cold conditions outside. E35 is a warning that your heating system may need attention before it fails completely.

Affected Models

Nest Learning Thermostat (2nd, 3rd gen)
Nest Thermostat E
Google Nest Thermostat (2020)

Common Causes

A severely clogged air filter is restricting airflow, preventing warm air from reaching the living spaces
A cracked or disconnected duct is losing heated air into the attic, crawlspace, or walls before it reaches the rooms
The furnace heat exchanger is failing and the furnace is shutting down on safety lockout before fully heating the air
In heat pump mode, outdoor temperatures have dropped below the system's effective operating range (typically around 35°F/2°C)
The Nest is located in a cold spot or near a drafty window and is reading a lower temperature than the rest of the home

How to Fix It

Inspect and replace your air filter immediately. A clogged filter restricts heated air so severely that the furnace may overheat and shut down repeatedly — causing E35. Replace if it has been more than 2 months since the last change.

Hold the filter up to a light. If you cannot see light through it, it needs to be replaced. This is the single most common cause of heating performance errors.
Walk around your home and check that all supply and return air vents are fully open and unobstructed. Blocked or closed vents create pressure imbalances that prevent proper heat distribution.

Furniture, rugs, or curtains placed over vents are a common cause of poor heat distribution. Make sure there is at least 12 inches of clearance around each vent.
If you have a heat pump, check the outdoor unit for ice buildup. A heat pump covered in ice cannot absorb heat from the outside air. Run the defrost cycle manually if available, or set the system to 'Emergency Heat' mode temporarily.

A light frost on a heat pump is normal. A solid block of ice that does not melt after defrost indicates a problem with the defrost cycle and needs a technician.
Listen to your furnace while it is running. If it turns on, runs for a short time (less than 5 minutes), and then shuts off repeatedly, this is called short-cycling. Check the filter and vents — if those are clear, call a technician.

Short-cycling is the furnace's safety system shutting it down due to overheating or a sensor fault. Continuing to run a short-cycling furnace risks damage.
Dismiss the E35 error in the Nest app and check the energy history for heating runtimes. Very long runtimes with slow temperature rise confirms reduced heating capacity. Compare to previous winters — a sharp increase in runtime indicates a degrading system.

In the Nest app, go to Energy > Heating to see runtime history by day or month.

When to Call a Professional

A furnace heat exchanger problem is a safety issue — a cracked heat exchanger can leak carbon monoxide into your home. If your furnace is short-cycling (turning on and off frequently) alongside E35, call an HVAC technician promptly. A carbon monoxide detector is essential in any home with a gas furnace. Expect to pay $75 to $150 for an HVAC diagnostic visit.

Frequently Asked Questions

Is E35 a gas leak warning?

No. E35 is specifically about the temperature not rising fast enough — it is a performance warning, not a gas or CO alert. However, a cracked heat exchanger (which can cause E35 through short-cycling) is also a source of carbon monoxide. If you smell gas or your CO detector activates, leave the home immediately and call 911 and your gas company — do not wait to troubleshoot. For E35 without any smell or alarm, you have time to investigate safely.

What temperature range can a heat pump handle?

Standard heat pumps are efficient down to about 35°F to 40°F (2°C to 4°C). Below that, they struggle to extract enough heat from the outside air and efficiency drops sharply. Modern 'cold-climate' heat pumps (like the Mitsubishi Hyper-Heat) can operate effectively down to -13°F (-25°C). If you live in a cold climate and your standard heat pump triggers E35 during cold snaps, your home may benefit from a heat pump upgrade or a supplemental heat source.

How can I prevent E35 from appearing every winter?

Annual HVAC maintenance is the best prevention. Have a technician inspect and tune your furnace or heat pump each autumn before heating season starts. They will check for duct leaks, test the heat exchanger, clean the burners, and check refrigerant levels on heat pumps. Regular filter changes every 1 to 3 months also prevent the majority of heating performance errors.